A spokesman for the Amsterdam police said: "At the end of the afternoon, a large group of Celtic supporters attacked police officers in plain clothes. Eight were injured and one was knocked unconscious. A few of them had broken noses and needed stitches above their eyebrows and on their lips. Bottles and sticks were used in the attack, which came out of nowhere."

A huge police presence in Amsterdam on Wednesday afternoon had included teams of plain-clothes officers amid fears of prearranged trouble.

Celtic issued a statement on Thursday evening. The club also referred to the attack on a bar full of Celtic fans by Ajax supporters on Tuesday and disorder caused by the Dutch contingent when the teams met in Glasgow.

Celtic's statement read: "Celtic Football Club is investigating events in Amsterdam. We have been informed that there are five individuals detained in Holland. We are not clear if they have any formal association with the club, however, we will be liaising with the local authorities in Amsterdam, our own police and the British Consulate in order to fully understand events. We would stress that there is no excuse for any form of violent conduct.

"In addition, we have received numerous complaints from Celtic supporters in relation to their treatment in Amsterdam, which we will also investigate fully. There is no question that Celtic supporters have been subjected to a high degree of provocation. Our supporters were the target of missiles being thrown from Ajax supporters at Celtic Park, a night where 300 seats were damaged within the stadium.

"This was followed up by an organised attack on a group of Celtic supporters in Amsterdam on Tuesday, the night before the match. We have also received numerous individual reports of other attacks on our fans. The Celtic travelling support has a magnificent reputation across Europe and our fans have visited major European cities over many years without incident. Once again the overwhelming majority of our supporters behaved impeccably."